VoltageDrop Tip

Voltage drop calculator

Enter the voltage, the load current and the one-way length of the run, then pick the wire. VoltageDrop calculates the voltage drop in volts and percent, the voltage left at the load and the power lost as heat, and suggests the smallest wire that keeps the drop within your limit and can carry the current. The example is a 120-volt, 20-amp circuit running 100 feet on 12 AWG copper, which drops 6.6% — too much.

Copper or aluminum, AWG / kcmil

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    Resistance values are the NEC Chapter 9 Table 8 figures for stranded conductors at 75 °C, the same data electricians use for hand calculations.

    How the voltage drop calculator works

    VoltageDrop uses VD = 2 × I × R × L ÷ 1000 for DC and single-phase circuits and VD = 1.732 × I × R × L ÷ 1000 for three-phase, where I is the current in amps, R the conductor resistance in ohms per 1000 ft and L the one-way length in feet. Percent drop = VD ÷ supply voltage × 100. The suggested wire is the smallest size whose drop is within your limit and whose ampacity covers the current (125% of it for a continuous load).

    Wire gauge chart

    Sizemm²Copper Ω/1000 ftAluminum Ω/1000 ftCopper amps (75 °C)Aluminum amps (75 °C)
    18 AWG0.8237.95———
    16 AWG1.314.99———
    14 AWG2.083.14—20 (15 A breaker)—
    12 AWG3.311.983.2525 (20 A breaker)20 (15 A breaker)
    10 AWG5.2611.242.0435 (30 A breaker)30 (25 A breaker)
    8 AWG8.3670.7781.285040
    6 AWG13.30.4910.8086550
    4 AWG21.150.3080.5088565
    3 AWG26.670.2450.40310075
    2 AWG33.620.1940.31911590
    1 AWG42.410.1540.253130100
    1/0 AWG53.490.1220.201150120
    2/0 AWG67.430.09670.159175135
    3/0 AWG85.010.07660.126200155
    4/0 AWG107.20.06080.1230180
    250 kcmil1270.05150.0847255205
    300 kcmil1520.04290.0707285230
    350 kcmil1770.03670.0605310250
    400 kcmil2030.03210.0529335270
    500 kcmil2530.02580.0424380310

    Resistance: NEC Chapter 9 Table 8, stranded uncoated copper and aluminum, DC at 75 °C. Ampacity: NEC Table 310.16, 75 °C column (not more than three current-carrying conductors, 30 °C ambient); breaker limits for 14–10 AWG from NEC 240.4(D). 18 and 16 AWG are shown for low-voltage DC only.

    Limits of this calculator

    Results are estimates for planning. The calculation uses DC resistance and ignores reactance and power factor, conductor temperature below 75 °C, and derating for heat, conduit fill or bundling. Terminal temperature ratings, insulation type, local codes and the equipment instructions can all require a larger wire. Have a licensed electrician design and inspect mains-voltage work.

    Frequently asked questions

    What is an acceptable voltage drop?

    The NEC informational notes suggest no more than 3% on a branch circuit and 5% in total for feeder plus branch circuit. It is a recommendation, not a requirement, except for some special equipment.

    What is the voltage drop formula?

    Single-phase and DC: VD = 2 × K × I × L ÷ CM, or 2 × I × R × L ÷ 1000 with R in ohms per 1000 ft. Three-phase: replace 2 with 1.732.

    Is the length one way or round trip?

    Enter the one-way distance from the source to the load. The formula doubles it for the return conductor.

    Is VoltageDrop free?

    Yes, with no sign-up or ads.

    Are my inputs saved?

    Yes, in this browser only.

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